The present invention concerns cooling fans, such as fans driven by and for use in cooling an industrial or automotive engine. More particularly, certain aspects of the invention relate to a ring fan, while other features concern fan blade design.
In most industrial and automotive engine applications, an engine-driven cooling fan is utilized to draw air through a coolant radiator. Usually, the fan is driven through a belt-drive mechanism connected to the engine crankshaft.
A typical cooling fan includes a plurality of blades mounted to a central hub plate. The hub plate can be configured to provide a rotary connection to the belt drive mechanism, for example. The size and number of fan blades is determined by the cooling requirements for the particular application. For instance, a small automotive fan may only require four blades having a diameter of only 9″. In larger applications, a greater number of blades is required. In one typical heavy-duty automotive application, nine blades are included in the fan design, the blades having an outer diameter of 704 mm.
In addition to the number and diameter of blades, the cooling capacity of a particular fan is also governed by the airflow volume that can be generated by the fan at its operating speed. This airflow volume is dependent upon the particular blade geometry, such as the blade area and curvature or profile, and the rotational speed of the fan.
As the cooling fan dimensions and airflow capacity increase, the loads experienced by the fan, and particularly the blades, also increase. In addition, higher rotational speeds and increased airflow through the fan can lead to de-pitching of the blades and significant noise problems. In order to address these problems to some degree, certain cooling fan designs incorporate a ring around the circumference of the fan. Specifically, the blade tips are attached to the ring, which provides stability to the blade tips. The ring also helps reduce vortex shedding at the blade tip, particularly when the ring is combined with a U-shaped shroud that follows the circumference of the ring.
The ring fan design, therefore, eliminates some of the structural difficulties encountered with prior unsupported cooling fan configurations. However, with the increased strength and improved vibration characteristics provided by the ring fan, the nominal operating conditions for these fans have been increased to again push the envelope of the ring fan's capability. Moreover, the mass inertia of the circumferential ring increases the centripetal force exerted on the blade-ring interface. Thus, similar to prior cooling fan designs, there is a limit to the amount of force that can be exerted on the ring fans before they fail. For plastic or fiber reinforced plastic molded ring fans, which are formed by injection molding, failure typically due to stress occurs along weld lines or knit lines, which are formed wherein two opposing flow fronts of molten polymeric material collide “head-on” substantially at an 180 degree angle relative to each other during the molding process.
Consequently, a need has again developed for ways to improve cooling airflow capacity of ring fans, while at the same time increasing their strength. This need becomes particularly acute as the operational rotational speeds of the fan increase to meet the increasing cooling demands for large industrial and automotive engines.